This invention relates, in general, to methods of making blend compositions of an unmodified polyvinyl alcohol and a metallocene polyolefin or grafted metallocene polyolefin and thermoplastic film and fiber structures comprising these blend compositions. More specifically, this invention relates to methods of making substantially water-free films and fibers comprising unmodified polyvinyl alcohol and a metallocene polyolefin or grafted metallocene polyolefin.
Personal care articles are widely used in today""s society. Many of these articles use films and fibers that are thermoplastic. Additionally, these articles use films and fibers that have different properties, depending on their location in the product. For example, some films and fibers are elastomeric. Others are breathable while still others act as liquid barriers. Finally, some of the films and fibers, especially those in contact with the wearer of the product, are designed to be softer to the touch. These different films typically comprise polymers or polymer blends that, when processed, form a film or fiber having the desired characteristic or characteristics.
Additionally, in an attempt to deal with decreasing land-fill and solid waste disposal many of these films and fibers are designed to be water-dispersible such that the product will partially or completely disperse in water, thereby allowing the product to be disposed of without dumping or incineration. These products may be placed in sewage systems or may be flushed down a conventional toilet. To produce these water-dispersible products, the films and fibers used in the products will typically use blend compositions that include a water-dispersible polymer such as polyethylene oxide or polyvinyl alcohol.
Polyvinyl alcohol (PVOH) is a commodity polymer that is used in a wide variety of different applications. Many of these applications are thermoplastic. However, PVOH is generally regarded as a non-thermoplastic polymer. PVOH has a high melting point of about 200xc2x0 C. depending on the degree of hydrolysis. Accordingly, as PVOH is heated near its melting point, yellowing and discoloration occur. Therefore, when using PVOH as a base material for thermoplastic applications, the PVOH must usually be modified.
Modified PVOH is used in many different water-dispersible thermoformable articles, such as fibers, films and fabrics which maintain their integrity and strength when in use, but dissolve and disperse when placed in contact with water. Unmodified PVOH is used in industry for many different solution-based applications and is not generally considered to be thermoformable or melt-processable. Some such applications for unmodified PVOH include warp sizing in textiles, fabric finishing, adhesives, paper processing additives, and emulsifiers/dispersants.
The prior art has demonstrated some success in modifying PVOH for use in thermoplastic applications. By xe2x80x9cmodifiedxe2x80x9d PVOH, it is meant PVOH resin which has been chemically modified, including PVOH having another compound grafted thereto, or PVOH resin that has been mixed with one or more plasticizers. In each instance, these xe2x80x9cmodificationsxe2x80x9d have been needed to permit PVOH to be used in thermoformable articles.
To overcome the thermoplastic processing problems, chemically modified PVOH has been used. Some prior art teachings have used ethers of PVOH, ethoxylated PVOH or lacton-modified PVOH to produce thermoformable articles.
The prior art has also used PVOH that has not been modified structurally by adding a plasticizing agent to the PVOH which permits the PVOH to be extruded into films and fibers. Examples of plasticizers include water, ethylene glycol, glycerin and ethanolamine.
However, there are problems associated with the addition of plasticizers to PVOH. One of the most pronounced problems during processing is the fogging of the volatile plasticizer during the melt extrusion and condensing of vapor and effects of the vapor to the operating environment. In addition, the extruded articles such as films or fibers lose the plasticizers since the plasticizer molecules diffuse out of the film or fibers. This causes the films or fibers to become brittle over time and often causes the article to fail.
Additionally, films and fibers including modified PVOH or PVOH and a plasticizer may be limited in their utility. These films and fibers may be too stiff to be used for certain applications. Additionally, the texture of the films may not be soft enough for comfortable contact with the skin of an individual.
Accordingly, what is needed is an unmodified PVOH that may be used in blend compositions that are thermoplastically formed into films and fibers. These films and fibers may then be used in the production of water-dispersible, flushable articles without the use of plasticizing agents. These fibers, films and fabrics could be used in products such as personal care products, diapers, feminine napkins and pads, training pants, wipes, adult incontinence products, release liners, product packaging, etc., which contain the above-mentioned fibers, films and fabrics. Additionally, what is needed are methods of making thermoplastic films and fibers that have enhanced softness and ductility.
Accordingly, the present invention desires to produce films and fibers including blend compositions having unmodified PVOH and a metallocene polyolefin or grafted metallocene polyolefin.
Another desire of the present invention is to use unmodified PVOH and a metallocene polyolefin or grafted metallocene polyolefin to form films and fibers without the use of a plasticizing agent.
These and other desires are satisfied by the present invention. The present invention discloses the selection and use of commercially-available grades of PVOH for thermoplastic applications. xe2x80x9cThermoplasticxe2x80x9d is defined, herein, as a resin which can be melted and easily extruded to form a desired article, i.e., the material is melt processable. These commercially-available grades of PVOH are combined with a metallocene polyolefin or grafted metallocene polyolefin to provide a blend composition useful in the production of films and fibers that have enhanced softness and ductility.
PVOH is a commodity polymer, commonly used in aqueous solution-based applications. Since it is a commodity polymer, thermoplastic articles made using unmodified PVOH are generally less expensive than articles made using modified PVOH due to the additional process steps required to modify the PVOH. Also, unmodified PVOH is, in general, less expensive than other water-soluble polymers.
In its unmodified form, PVOH has not been used for thermoplastic applications. Typically, some modification of the PVOH, such as chemical grafting or addition of plasticizer, is necessary to achieve melt processability for PVOH. In the present invention, a window of thermoplastic processability has been discovered and defined for unmodified, commercially-available PVOH, according to: 1) the composition or % hydrolysis of the PVOH, 2) the molecular weight of the PVOH, 3) the solution viscosity of the PVOH, or 4) the melt viscosity of the PVOH. The selected grades of PVOH have demonstrated thermoplasticity, allowing for continuous, melt extrusion or conversion into thin films in a continuous, extrusion process.
These grades of PVOH are also useful for melt spinning of fibers, injection molding or other thermoplastic applications. Extruded films of the unmodified PVOH/metallocene polyolefin or grafted metallocene polyolefin blends described herein have very high strength and modulus, excellent clarity, and fast crystallization and solidification rates. The advantages of melt processing a thermoplastic, unmodified PVOH into a useful, strong, clear, water-soluble article are evident. Melt processing is a desirable thermoforming process compared to solution processing. Melt processing eliminates the need to add steps such as chemical grafting, addition of a plasticizer, or other modification in order to achieve melt processability.
These grades of PVOH may be mixed with additional polymers, such as metallocene polyolefins or grafted metallocene polyolefins, to provide desired characteristics to the films and fibers, such as enhanced ductility and enhanced softness.
PVOH is generally produced by a two step process as shown in Scheme 1. Since vinyl alcohol is not a stable monomer, the polymerization of vinyl alcohol is not an option for making PVOH. Instead, the process utilizes a readily available monomer, vinyl acetate, as the starting point. The first step is the polymerization of vinyl acetate into polyvinyl acetate (PVA). The second step is the hydrolysis or alcoholysis of PVA into a copolymer of vinyl acetate and vinyl alcohol, or polyvinyl alcohol (PVOH). Depending on the hydrolysis level as defined in the equation in Scheme 1, a wide range of PVOH copolymers can be produced when the hydrolysis reaction is allowed to reach certain conversion levels. 
For PVOH, the degree of hydrolysis is controlled during the alcoholysis reaction and is independent of the control of the molecular weight of the PVOH formed. Fully hydrolyzed PVOH is obtained if alcoholysis is allowed to go to completion. The reaction is terminated by removing or neutralizing the sodium hydroxide catalyst used in the process. Typically, a small amount of water is added to the reaction vessel to promote the saponification reaction of PVA. The extent of hydrolysis is inversely proportional to the amount of water added. The alcoholysis can be carried out in a highly agitated slurry reactor. A fine precipitate forms as PVA, which is then converted to PVOH. The PVOH product is then washed with methanol and is filtered and dried to form a white, granular powder.
The molecular weight of the PVOH is controlled by the polymerization condition of vinyl acetate. Many properties of PVOH depend on the degree of hydrolysis and the molecular weight. As the molecular weight increases, the solution viscosity, tensile strength, water resistance, adhesive strength, and solvent resistance increase. As molecular weight decreases, the flexibility, water solubility, and ease of solvation increase. As the degree of hydrolysis increases, the water resistance, tensile strength, block resistance, solvent resistance, and adhesion to polar substrates increase. As the degree of hydrolysis decreases, the water solubility, flexibility, water sensitivity and adhesion to hydrophobic substrates increase.
Due to the strong dependence of PVOH on the molecular weight and degree of hydrolysis, PVOH is typically supplied in combination of these two parameters. PVOH is classified into 1) partially hydrolyzed (87.0 to 89.0% hydrolysis); 2) intermediately hydrolyzed (95.5 to 96.5% hydrolysis); 3) fully hydrolyzed (98.0 to 98.8% hydrolysis); and 4) super hydrolyzed ( greater than 99.3% hydrolysis). Within each category of PVOH, the resin is differentiated by solution viscosity, measured at 4% solution in water at 20xc2x0 C. in centipoise. The viscosity is used as a molecular weight measure since solution viscosity is typically related to the molecular weight by the well known Mark-Houwink equation:
xcex7=KMva
wherein
xcex7=intrinsic viscosity
K=constant (dependent upon the polymer)
Mv=molecular weight
a=factor based on the rigidity of the polymer chains and is dependent on the polymer.
For unmodified PVOH, it was known that higher molecular weight grades were not thermoplastic. It was surprising that unmodified PVOH at lower molecular weights would be thermoplastic based on the non-melt processability of higher molecular weights grades. Unmodified PVOH with weight average molecular weight as low as 8750 g/mole was discovered to be thermoplastic and melt processable, with high melt strength, excellent film strength and great clarity. Typically, a polymer with such a low starting molecular weight would not be expected to be melt processable into a useful material.
Additionally, it was discovered that the melt viscosity of the PVOH grades could be used to determine which grades of PVOH were thermoplastic. In general, those grades having a melt viscosity less than about 1500 Paxc2x7s at a shear rate of 500 sxe2x88x921 were determined to be melt processable.
Not all grades of PVOH were discovered to be thermoplastic. The PVOH grades useful in this invention desirably have a solution viscosity of less than about 10 cp in a 4% water solution at 20xc2x0 C. and a hydrolysis of less than about 90%. Examples of commercially-available grades of PVOH useful in this invention are ELVANOL(copyright) 51-05 from DuPont (Wilmington, Del.), AIRVOL(copyright) 203 and 205 from Air Products and Chemical, Inc. (Allentown, Pa.), and GOHSENOL(copyright) KP-06 from Nippon Gohsei (Japan). PVOH is typically sold in powder or granule form, however pellets or other forms of resin can be used in this invention since the physical form of PVOH does not affect melt processability.
Metallocene polyolefins (mPO) are the polyolefins manufactured by polymerizing olefinic monomers by metallocene catalysts. The metallocene polyolefins have better controlled polymer microstructures than the polyolefins manufactured by using conventional Ziegler-Natta catalysts, including narrower molecular weight distribution, well-controlled composition distribution, comonomer sequence distribution, and stereoregularity. Metallocene polyolefins include both the homo-polymers of ethylene, propylene, and alpha-olefins containing up to 20 carbon atoms and co-polymers thereof with other alpha-olefins and functional monomers. Examples of metallocene polyolefins include, but are not limited to, the EXACT(copyright) polyolefins from Exxon Mobil Chemicals (Houston, Tex.) and AFFINITY(copyright) plastomers by Dow Chemical Company (Midland, Mich.).
Additionally, depending on the type of blend application for which the PVOH will be used, films or fibers, the exact processing characteristics may vary. For example, some of the thermoplastic grades may be better suited for the production of thermoplastic films while other grades may be more useful for the production of fibers. The exact grade to use will depend upon the item being made and the metallocene polyolefin or grafted metallocene polyolefin that is blended with the PVOH.
The present invention uses these thermoplastic PVOH grades with an additional compound to form blend compositions. These blend compositions may then be formed into thermoplastic articles such as films and fiber using the methods of the present invention. The additional compound is used to enhance the properties of the resulting films and fibers. In the present invention, a metallocene polyolefin or grafted metallocene polyolefin is used to help produce films that are softer and more ductile than films comprising PVOH alone. The present invention is able to achieve these results even though PVOH and metallocene polyolefins are generally incompatible. However, the metallocene polyolefins or grafted metallocene polyolefins used in the methods of the present invention have improved compatibility with PVOH.
A variety of monomers may be useful in the practice of this invention. The term xe2x80x9cmonomer(s)xe2x80x9d as used herein includes monomers, oligomers, polymers, mixtures of monomers, oligomers and/or polymers, and any other reactive chemical species that are capable of covalent bonding with the parent polymer, metallocene polyolefins. Suggested monomers are ethylenically unsaturated and contain a polar vinyl group. Such monomers are termed xe2x80x9cpolar vinylxe2x80x9d herein. A variety of polar vinyl monomers may be useful in the practice of this invention. Monomer as used herein includes monomers, oligomers, polymers, mixtures of monomers, oligomers and/or polymers, and any other reactive chemical species that is capable of covalent bonding with the parent polymer, metallocene polyolefin. Methods of grafting polyolefin compositions and particularly desirable grafted-polyefin compositions are disclosed in U.S. patent application Ser. No. 08/733,410 filed Oct. 18, 1996, now U.S. Pat. No. 6,707,405 the disclosure of which is herein incorporated in its entirety. The methods are applicable to mPO as well.
Ethylenically unsaturated monomers containing a polar functional group, such as hydroxyl, carboxyl, amino, carbonyl, halo, thiol, sulfonic, sulfonate, etc. are appropriate for this invention and are desired. Ethylenically unsaturated polar monomers include 2-hydroxyethyl methacrylate (hereinafter HEMA), poly(ethylene glycol) methacrylates (hereinafter PEG-MA) including poly(ethylene glycol) ethyl ether methacrylate, poly(ethylene glycol) acrylates, poly(ethylene glycol) ethyl ether acrylate, poly(ethylene glycol) methacrylates with terminal hydroxyl groups, acrylic acid, maleic anhydride, itaconic acid, sodium acrylate, 3-hydroxypropyl methacrylate, acrylamide, glycidyl methacrylate, 2-bromoethyl acrylate, carboxyethyl acrylate, methacrylic acid, 2-chloroacrylonitrile, 4-chlorophenyl acrylate, 2-cyanoethyl acrylate, glycidyl acrylate, 4-nitrophenyl acrylate, pentabromophenyl acrylate, poly(propylene glycol) methacrylate, poly(propylene glycol) acrylate, 2-propene-1-sulfonic acid and its sodium salt, sulfo ethyl methacrylate, 3-sulfopropyl methacrylate, and 3-sulfopropyl acrylate.
Desired ethylenically unsaturated monomers include acrylates and methacrylates. Particularly desirable monomers, oligomers, polymers, mixtures of monomers, oligomers and/or polymers, and any other reactive chemical species which is capable of covalent bonding with the parent polymer, metallocene polyolefin, ethylenically unsaturated monomers containing a polar functional group are 2-hydroxyethyl methacrylate (hereinafter HEMA) and poly(ethylene glycol) methacrylates (hereinafter PEG-MA). A particularly desirable poly(ethylene glycol) methacrylate is poly(ethylene glycol) ethyl ether methacrylate. However, it is expected that a wide range of polar vinyl monomers would be capable of imparting similar effects as HEMA and PEG-MA to metallocene polyolefin (mPO) and would be effective monomers for grafting. For grafting purposes, the amount of polar vinyl monomer relative to the amount of mPO may range from about 0.05 to about 30 weight percent of monomer to the weight of mPO. Desirably, the amount of monomer should exceed 0.1 weight percent to improve the processability of the mPO. A range of grafting levels is demonstrated in the above-incorporated U.S. patent applications. Typically, the monomer addition levels were between 2.5 percent and 15 percent of the weight of the base mPO resin. Ethylenically unsaturated monomers containing a polar functional group, such as hydroxyl, carboxyl, amino, carbonyl, halo, thiol, sulfonic, sulfonate, etc. are appropriate for this invention and are desired. Desired ethylenically unsaturated monomers include acrylates and methacrylates. It is expected that a wide range of polar vinyl monomers would be capable of imparting similar effects as HEMA and PEG-MA and would be effective monomers for grafting.
This invention has been demonstrated in the following Examples by the use of HEMA as the polar vinyl monomer. The HEMA was obtained from Aldrich Chemical Company and is designated Aldrich Catalog number 12,863-5. The grafted mPE used in the following Examples was an EXACT(copyright) 4151 metallocene polyethylene, grafted with 9 weight percent HEMA and 0.13, 0.28, or 0.54 weight percent LUPERSOL(copyright) 101 initiator, respectively. The process temperature was 180xc2x0 C. and the screw speed was 300 rpm using the twin screw extruder set forth in Example 3, hereinbelow. PEG-MA is also a suggested monomer and can also be obtained from Aldrich Chemical Company. A desirable PEG-MA is poly(ethylene glycol) ethyl ether methacrylate, sold under the Aldrich Catalog designation number 40,954-5. Poly(ethylene glycol) ethyl ether methacrylate is a derivative of poly(ethylene methacrylate). The poly(ethylene glycol) ethyl ether methacrylate sold by Aldrich under the above designation number has a number average molecular weight of approximately 246 grams per mol. PEG-MA with a number average molecular weight higher or lower than 246 g/mol are also applicable for this invention. The molecular weight of the PEG-MA can range up to 50,000 g/mol. However, lower molecular weights are preferred for faster grafting reaction rates. The desired range of the molecular weight of the monomers is from about 246 to about 5,000 g/mol and the most desired range is from about 246 to about 2,000 g/mol. Again, it is expected that a wide range of polar vinyl monomers as well as a wide range of molecular weights of monomers would be capable of imparting similar effects to mPO resins and blends incorporating such grafted-mPE resins and would be effective monomers for grafting and modification purposes.
A variety of initiators may be useful in the grafting of the mPO. When grafting is achieved by the application of heat and intensive mixing, as in a reactive-extrusion process, it is desirable that the initiator generates free radicals through the application of heat. Such initiators are generally referred to as thermal initiators. For the initiator to function as a useful source of radicals for grafting, the initiator should be commercially and readily available, stable at ambient or refrigerated conditions, and generate radicals at reactive-extrusion temperatures.
Compounds containing an Oxe2x80x94O, Sxe2x80x94S, or Nxe2x95x90N bond may be used as thermal initiators. Compounds containing Oxe2x80x94O bonds, peroxides, are commonly used as initiators for polymerization. Such commonly used peroxide initiators include: alkyl, dialkyl, diaryl and arylalkyl peroxides such as cumyl peroxide, t-butyl peroxide, di-t-butyl peroxide, dicumyl peroxide, cumyl butyl peroxide, 1,1-di-t-butyl peroxy-3,5,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne-3 and bis(a-t-butyl peroxyisopropylbenzene); acyl peroxides such as acetyl peroxides and benzoyl peroxides; hydroperoxides such as cumyl hydroperoxide, t-butyl hydroperoxide, p-methane hydroperoxide, pinane hydroperoxide and cumene hydroperoxide; peresters or peroxyesters such as t-butyl peroxypivalate, t-butyl peroctoate, t-butyl perbenzoate, 2,5-dimethylhexyl-2,5-di(perbenzoate) and t-butyl di(perphthalate); alkylsulfonyl peroxides; dialkyl peroxymonocarbonates; dialkyl peroxydicarbonates; diperoxyketals; ketone peroxides such as cyclohexanone peroxide and methyl ethyl ketone peroxide. Additionally, azo compounds such as 2,2xe2x80x2-azobisisobutyronitrile abbreviated as AIBN, 2,2xe2x80x2-azobis(2,4-dimethylpentanenitrile) and 1,1xe2x80x2-azobis(cyclohexanecarbonitrile) may be used as the initiator. This invention has been demonstrated in the following Examples by the use of a liquid, organic peroxide initiator available from Elf Atochem North America, Inc. of Philadelphia, Pa., sold under the trade designation LUPERSOL(copyright) 101. LUPERSOL(copyright) 101 is a free radical initiator and comprises 2,5-dimethyl-2,5-di(t-butylperoxy) hexane. Other initiators and other grades of LUPERSO(copyright) initiators may also be used, such as LUPERSOL(copyright) 130.
The grafting of mPO may be performed in a continuous reaction device, such as an extruder. A twin screw extruder is preferred for grafting polar monomers onto mPO due to its high intensity of dispersive and distributive mixing. The level of grafting of mPO ranges from about 1 to about 30% by weight. Desirably, the level of grafting is from about 2 to about 20% by weight. Even more desirably, the level of grafting is from about 3 to about 15% by weight.
The blends including thermoplastic PVOH grades and a metallocene polyolefin or grafted metallocene polyolefin may be extruded using most known extruding devices. In general, while a thermoplastic film may be extruded at extrusion temperatures above the melting point of the PVOH/metallocene polyolefin blend, it is preferred to use extrusion temperatures near the melting point as the resulting films and fibers are generally clearer, have fewer imperfections, are more ductile and stronger, and can be drawn into much thinner films.
As discussed earlier, the films and fibers made by the methods of the present invention can be extruded from unmodified PVOH/metallocene polyolefin blends without the use of a plasticizer. Many different plasticizers are known, including, for example, ethylene glycol, glycerines and ethanolamine. In addition to these plasticizers, water is also known to be used as a plasticizer in the production of PVOH films and fibers. However, these plasticizers, including water, have several disadvantages when used in the production of films and fibers. In general, plasticizers, including water, will slowly diffuse out of a PVOH film or fiber causing the film or fiber to become lucid and brittle and therefore more likely to break or shatter.
Additionally, plasticizers, including water, added to PVOH may cause bubbling of the filmri during the extrusion process. This is especially true with water. Therefore, care must be taken prior to the blending with a metallocene polyolefin or grafted metallocene polyolefin and production of the film to ensure that the PVOH powder or pellets remain substantially water-free. This helps to ensure that the films and fibers produced by the methods of the present invention are also substantially water-free. By xe2x80x9csubstantially water-freexe2x80x9d it is meant that the films and fibers produced using the methods of the present invention contain less than about 2.0 percent by weight of water. Desirably, the films and fibers contain less than about 1.0 percent by weight of water. More desirably, the films and fibers contain less than 0.5 percent by weight of water.
The importance of this invention is that PVOH/metallocene polyolefin blends have been discovered that may be directly extruded into a water-soluble, thin film without the need for any chemical modification of the PVOH or the addition of a plasticizer. The elimination of any chemical modification of the PVOH eliminates the labor intensive step of chemically modifying or grafting the PVOH. The elimination of a plasticizer admixed with the PVOH relieves the common problems involved with plasticizers as previously discussed. The water-soluble film made by the present invention will keep its original properties and in-use performance unlike a PVOH/metallocene polyolefin film containing a plasticizer which will become brittle over time.
One additional advantage in the production of water-soluble products from the PVOH/metallocene polyolefin films and fibers is in the product converting stage. PVOH has a higher melting point than many other water-soluble polymer systems used for making water-dispersible, flushable articles, including, for example, polyethylene oxide-based materials. PVOH film can withstand heat from a hot-applied melt adhesive which may be used during product construction. In contrast, PEO-based materials have limitations in this aspect due to the low melting temperature of the PEO of about 60 to 70xc2x0 C. Therefore, the PVOH/metallocene polyolefin films and fibers made by the present invention have great usefulness in the production of water-dispersible, flushable products.
The PVOH/metallocene polyolefin blends, films and fibers made by the methods of the present invention include a metallocene polyolefin or grafted metallocene polyolefin that enhances certain characteristics of the films and fibers when compared to films and fibers comprising only unmodified PVOH. The metallocene polyolefin or grafted metallocene polyolefin imparts improved softness and ductility to the film. These features are very useful for films that are used in a personal care article, such as a diaper, feminine article, incontinence device, among others.
The amount of metallocene polyolefin or grafted metallocene polyolefin that may be used is in the amount of from about 1 to about 99% by weight of the PVOH/metallocene polyolefin blend. Desirably, the methods of the present invention use a blend comprises from about 50 to about 90% by weight PVOH and from about 50 to about 1% metallocene polyolefin or grafted metallocene polyolefin. Even more desirably, the blend comprises from about 65 to about 80% by weight PVOH and from about 35 to about 20% metallocene polyolefin or grafted metallocene polyolefin.